net/netvsc: process link change messages in alarm

net/netvsc: process link change messages in alarm

The original code would deadlock itself if a link change event
happened with link state interrupt enabled. The problem is that
the link state changed message would be seen while reading
the host to guest ring (under lock) and then the driver would
send a query to the host to see the new link state. The response
would never be seen (stuck in a while loop) waiting for the
response.

The solution is to use the link change indication to trigger
a DPDK alarm. The alarm will happen in a different thread and
in that context it can send request for new link state and
also do interrupt callback. This is similar to how the bonding
driver is handling the same thing.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>